1. Field of the Invention
The present invention relates to an optical device, a semiconductor laser module, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
Semiconductor laser devices have been used in various applications. Particularly, a large number of small-size semiconductor laser devices have been used in the optical communication field and in image forming apparatuses such as printers. In some semiconductor laser devices, a laser light emitted from the light-emitting part directly irradiates an optical system. However, in typical light scanning devices using many laser lights, laser lights are guided by means of optical fibers and emitted from the ends of optical fibers into the light scanning devices. This method serves to enhance the flexibility of arranging the semiconductor laser light source devices, thereby facilitating the design and manufacturing of the image forming apparatus including the light scanning device.
The combination including a semiconductor laser, an optical device coupling the laser light emitted from the semiconductor laser with an optical fiber, and the coupled optical fiber is called a semiconductor laser module. Semiconductor laser modules in which a laser light emitted from the semiconductor laser is effectively coupled with a single-mode optical fiber are widely used especially in the optical communication field. Generally, the core diameter of a single-mode optical fiber decreases as the oscillating wavelength of the semiconductor laser decreases. Namely, when a visible light semiconductor laser is used, the core diameter becomes several micrometers. Because of this feature, when a visible light is used in a semiconductor laser module, alignment between an incident light beam into an optical fiber and the optical fiber has to be performed much more accurately, and much higher structural stability of the module against environmental changes occurring after the manufacture of the module is also required compared with a case where an infrared light is used in a semiconductor laser module.
Further, since the incident end surface of the optical fiber in the semiconductor laser module is always irradiated by the laser light condensed into a spot of several micrometers, the optical energy density is focused on the incident end surface of the optical fiber. Under this circumstance, due to the optical tweezer effect (an effect of collecting dust on a light condensing point, see Non Patent Document 1), dust is disadvantageously adhered on the core portion of the end of the optical fiber. Unfortunately, in addition to the optical tweezer effect, with a short-wavelength laser having wavelength of 450 nm or less, the adhered dust and ambient gas easily produce a photochemical reaction with the laser light, thereby increasing the pollution of the light condensing part of the optical fiber.
Patent Document 1 describes the major pollutants of concern. One pollutant is generated by copolymerizing or decomposing the hydrocarbon gas, having intruded from the atmosphere during a manufacturing process, by reacting with the laser light. Another pollutant is siloxane floating in air, causing a photochemical reaction with laser light to generate SiOx that accumulates on and adheres to the light condensing part of the optical fiber. Because of this disadvantage, the exchange of a member possibly polluted is suggested.
According to, for example, Patent Documents 2 through 5, a method is provided in which a getter formed of a porous material or an organic adsorption agent is housed in the semiconductor laser module to adsorb pollutants such as an adhesive, a detergent, and flux for soldering that are used in a process of manufacturing a semiconductor laser module.
According to Patent Document 6, it is suggested that the package of the semiconductor laser module be hermetically sealed using flux-free solder or an adhesive free from silica-based organic matter or by way of fusion bonding or welding to prevent intrusion of pollutants into the package of the semiconductor module, thereby reducing the pollution of the incident end surface of the optical fiber.
On the other hand, to prevent the intrusion of impurities into the package of a semiconductor laser module, it is also suggested that an hermetically enclosed space be provided in the package. However, when, for example, the environment changes, an optical system in the semiconductor laser module may be distorted due to the pressure change in the package. The higher the required accuracy of condensing the light becomes, the more easily the efficiency of guiding the light fluctuates due to the distortion between the condensing light portion of the end surface of the optical fiber and the lens system of the laser chip and the optical system, thereby causing trouble in a subsequent process of the image forming apparatus.
To reduce the pressure change, Patent Document 7 discloses an optical device having a case whose volume can be changed. The optical device includes a case, made of a material that is not substantially deformed, having an opening and an elastic member, having a larger size than that of the opening, disposed on the opening. Because of this structure, when the volume of the case is changed, the volume of the elastic body is changed.
Patent Document 8 discloses an optical device having a variable volume mechanism changing a volume of the space on a light path. The variable volume mechanism enables maintaining the pressure of the space and, therefore, reduces the displacement or the deformation of the optical member when the amount of gas supplied into the optical device is changed.    Patent Document 1: Japanese Patent Application Publication No: H11-54852    Patent Document 2: Japanese Patent Application Publication No: H07-147457    Patent Document 3: Japanese Patent Application Publication No: H08-213678    Patent Document 4: Japanese Patent Application Publication No: H08-236660    Patent Document 5: Japanese Patent Application Publication No: 2002-544679    Patent Document 6: Japanese Patent Application Publication No: 2004-253783    Patent Document 7: Japanese Patent No: H03-27081    Patent Document 8: Japanese Patent Application Publication No: 2003-257821    Non Patent Document 1: Ashkin et al.: Observation of a single-beam gradient force optical trap for dielectric particles. Opt Lett. 11, PP. 288-290, 1986
In the inventions described in Patent Documents 2 through 6, since an optical fiber coupling system is completely isolated from outer air, the following problems may occur:
(1) When a semiconductor laser module is used at high or low altitude, the pressure difference between outer air and inside air deforms the optical fiber coupling system, thereby changing the amount of light guided into the optical fiber.
(2) When the pressure difference occurs between inside a semiconductor laser module and outer air, there is a danger that gas causing pollution may intrude through an incompletely sealed portion of the optical fiber coupling system.
(3) When humidity of outer air is high, moisture may intrude through an incompletely sealed portion of the optical fiber coupling system; condensation may be formed, thereby reducing the light use efficiency of the optical fiber coupling system.
Further, an object of Patent Document 7 is to provide a variable focal-distance optical device capable of changing the focal distance by treating the surface of the elastic member as the optical surface. But, the Patent Document 7 does not provide any specific suggestion with regard to the problem of the deformation of the optical system when the volume of the case is changed.
Also, the variable volume mechanism provided in Patent Document 8 moves slidably or elastically. There is, however, no description of the service life of the filter.